1. Field of the Invention
The disclosures herein generally relate to a memory circuit, and particularly relate to a nonvolatile memory circuit which is capable of retaining stored data in the absence of a power supply voltage.
2. Description of the Related Art
PermSRAM is a new type of nonvolatile semiconductor memory device, which uses a MIS (metal-insulating film-semiconductor) transistor as a nonvolatile memory cell (i.e., the basic unit of data storage). The MIS transistor used as a nonvolatile memory cell in PermSRAM has the same structure as ordinary MIS transistors used for conventional transistor functions (e.g., switching function), and do not require a special structure such as a floating gate or a special material such as a ferroelectric material or ferromagnetic material. The absence of such a special structure and special material offers an advantage in cost reduction, PermSRAM was initially disclosed in PCT/JP2003/016143, which was filed on Dec. 17, 2003, the entire contents of which are hereby incorporated by reference.
The MIS transistor used as a nonvolatile memory cell in PermSRAM is configured to experience an irreversible hot-carrier effect on purpose for storage of one-bit data. Here, the irreversible hot-carrier effect refers to the injection of carriers into the oxide film (insulating film) and/or sidewalls, which causes a change in the transistor's threshold voltage. Whether the threshold voltage has been changed due to a hot-carrier effect represents one-bit data “0” or “1”. Such a change in the threshold voltage may be detected by sensing a difference in ON current between the nonvolatile-memory-cell MIS transistor and a reference MIS transistor by using a sensing circuit such as a latch circuit.
Data supplied from an external source to a PermSRAM may be initially written to the latch circuit. A store operation is then performed to transfer the data from the latch circuit to the nonvolatile-memory-cell MIS transistor. Whether the nonvolatile-memory-cell MIS transistor experiences a hot-carrier effect in the store operation depends on whether the data stored in the latch circuit is 0 or 1. A recall operation is subsequently performed to read the data stored in the nonvolatile-memory-cell MIS transistor. If the nonvolatile-memory-cell MIS transistor has experienced a hot-carrier effect in the store operation, an ON current smaller than the ON current of the reference MIS transistor may be detected in the recall operation. If the nonvolatile-memory-cell MIS transistor has not experienced a hot-carrier effect in the store operation, an ON current larger than the reference ON current may be detected in the recall operation. The latch circuit may be used as a sense circuit to sense such difference in ON current, and latches data that is either “0” or “1” depending on whether the ON current of the nonvolatile-memory-cell MIS transistor is larger or smaller than the reference ON current.
Carriers trapped in the oxide film and/or sidewalls of a MIS transistor due to a hot carrier effect include both electrons and holes. In general, an electron having enough kinetic energy in a semiconductor material can knock a bound electron out of its bound state to create an electron-hole pair. This phenomenon is known as impact ionization. In a case of an NMOS transistor, hot electrons having high-kinetic energy flowing through a transistor channel from the source to the drain causes impact ionization to occur at the channel/drain junction to create electron-hole pairs. Such impact ionization also occurs in a PMOS transistor. That is, holes having high-kinetic energy causes impact ionization at the channel/drain junction of a PMOS transistor to create electron-hole pairs. Depending on the condition under which the transistor is driven, either electrons or holes become predominant carriers that are trapped in the oxide film and/or sidewalls of the MIS transistor as the result of a hot carrier effect.
If properly configured, PermSRAM may be able to select either electrons or holes as predominant carriers thereby to control a polarity of change in the threshold voltage.